Space propulsion, surface cleaning, ion implantation, and high energy accelerators use two or three closely spaced multiple-aperture electrodes to extractions from a source and eject them in a collimated beam. The electrodes are called "grids" because they are perforated with a large number of small holes in a regular array. Typically, the grids are made from molybdenum. A series of grids constitute an "ion optics" electrostatic ion accelerator and focusing system.
Ion beam sources designed for spacecraft propulsion, that is, ion thrusters, should have long lifetimes (10,000 hours or more), be efficient, and be lightweight. These factors can be important in other applications as well, but, for ion thrusters, they are critical. Ion thrusters have been successfully tested in space, and show promise for significant savings in propellant because of their high specific impulse (an order of magnitude higher than that of chemical rocket engines). They have yet to achieve any significant space use, however, because of lifetime limitations resulting from grid erosion and performance constraints resulting from thermal-mechanical design considerations, particularly the spacing of metallic grids, including molybdenum. We have discovered a way to extend the lifetime dramatically.
In an electron bombardment ion thruster, a cathode produces electrons that strike neutral gas atoms introduced through a propellant feed line. The electrons ionize the gas propellant and produce a diffuse plasma. In a radio frequency ion thruster, the propellant is ionized electromagnetically by an external coil, and there is no cathode. In both cases, an anode associated with the plasma raises its positive potential. To maintain the positive potential of the anode, a power supply pumps to ground potential some of the electrons that the anode collects from the plasma. These electrons are ejected into space by a neutralizer to neutralize the ion beam. Magnets act to inhibit electrons and ions from leaving the plasma. Ions drift toward the ion optics, and enter the holes in a screen grid. A voltage difference between the screen grid and an accelerator grid accelerates the ions, thereby creating thrust. The screen grid is at the plasma potential, and the accelerator grid is held at a negative potential to prevent downstream electrons from entering the thruster. Optionally, the optics can include a decelerator grid located slightly downstream of the accelerator grid and held at ground potential or at a lesser negative potential than the accelerator grid to improve beam focusing and reduce ion impingement on the negative accelerator grid.
Ion impact erosion of the ion optics (i.e., the grids) is the primary mechanism limiting the life of ion thrusters. In ion thrusters, slow moving ions are produced within and downstream of the ion optics by a charge exchange (i.e., electron hopping) from neutral propellant atoms to fast moving ions that pass close by. These "charge exchange" ions are attracted to the accelerator grid and strike it at high energy, gradually eroding it. The screen grid also experiences some erosion, mostly on the upstream side but generally only from plasma ions. The erosion of the accelerator grid eventually weakens it to the point that the grid fails and breaks.
A principal factor affecting both the efficiency and the weight of ion thrusters is how closely and precisely the grids can be positioned while maintaining relative uniformity in the grid-to-grid spacing at high operating temperatures or in conditions where the spatial temperature is nonuniform and thermal distortion can occur because of temperature gradients. In the past, this factor has limited the maximum practical diameter of ion thrusters, which severely constrains taking advantage of scale effects (that theoretically would improve efficiency), thrust-to-weight ratio, and reliability.
Molybdenum ion thruster grids are precisely hydroformed into matching convex shapes. The apertures are chemically etched in the formed sheets. The convex shapes provide a predictable direction for the deformation that occurs due to thermal expansion when a thruster heats in operation. Changes in the actual spacing and the uniformity of spacing over the grid surfaces between the molybdenum grids is unpredictable and uncontrollable. The thermal expansion distribution is complex.
The changes in spacing that occur adversely effect performance. Although techniques have been developed to compensate for such changes, the unpredictable and nonuniform nature of the changes prevents complete compensation.
In ion beam sources used for terrestrial applications, today's grids are sometimes made of graphite, which expands much less than molybdenum when heated. Graphite is, however, relatively flexible and fragile and is unsuitable for beam sources larger than about 15-20 cm in diameter, or for space applications where the ion thruster grids are subject to severe vibration during launch from Earth.
It is desirable to have a screen grid and accelerator grid that have lifetimes of 10,000 to 20,000 hours for use in a variety of space propulsion applications. Such grids should also have improved efficiency and should be lightweight. Additionally, the screen grids of an ion optics set should allow for precise prediction of the magnitude and uniformity of the spacing between the grids. The goal is to maintain the spacing over the temperature range and pattern of differential surface temperature that the grids experience.